Removal of Remazol Blue 19 from wastewater by zinc–aluminium–chloride-layered double hydroxides
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Layered double hydroxides (LDHs), also called anionic clays, consist of cationic brucite-like layers and exchangeable interlayer anions. These hydrotalcite-like compounds, with Zn and Al in the layers and chloride in the interlayer space, were prepared following the coprecipitation method at constant pH. The affinity of this material for Remazol Blue 19, RB19 [2-(3-(4-Amino-9,10-dihydro-3-sulpho-9,10-dioxoanthracen-4-yl) aminobenzenesulphonyl) vinyl) disodiumsulphate], was studied as a function of contact time, pH of the solutions LDH dose and the RB19/[Zn–Al–Cl] mass ratio. It was found that 48 h is enough time for the equilibrium state to be reached with maximum RB19 retention at pH of 9 for an LDH dose equal to 100 mg and with an RB19/[Zn–Al–Cl] mass ratio higher than 3. The adsorption isotherm, described by the Langmuir model, is of L-type. The results demonstrate that RB19 retention on LDHs occurs by adsorption on external surface when RB19/[Zn–Al–Cl] mass ratio is equal or <3 and by both adsorption and interlayer ion exchange for ratios higher than 3. A mechanism for removal of RB19 anion has been confirmed by X-ray diffraction, IR spectroscopy and TG analysis (TG and DTG curves).
KeywordsLayered double hydroxides Anionic clays Remazol Blue 19 Dye Adsorption Anion exchange Intercalation Retention
Industrial effluents from textile, tanneries or printing are polluting discharges that contain nonbiodegradable dyes (EL Gaini et al. 2008a). Their decontamination by conventional techniques seems ineffective in some cases.
The removal of colouring agents from textile effluents has attracted attention in the last few years, not only because of their toxicity, but also mainly due to their visibility. In recent years, many investigations have focused on several adsorbents. In the field of textile, special attention was paid to these industries by developing research to identify cost-effective methods of treatment of their releases coloured and meet required standards. A fair estimate of dye losses to the environment is about 1–2 % during their production and 1–10 % in their use (Forgacs et al. 2004). Improper disposal of dye-containing wastewaters that cause aesthetic problems concerns not only the scientists, but is now beginning also to draw the public authority’s attention and can generate toxic effects to aquatic life. Effluents from textile dyeing are constituted by complex mixtures of dyes, auxiliary chemicals, salts, acids, bases, organochlorines and sometimes heavy metals (Goncalves et al. 2000).
The removal of coloured contaminants is one of the main problems of treating this type of effluents, because of resistance to the biodegradability of dyeing, light, heat and oxidants (Sun and Yang 2003). Traditional treatments involving biological (Walker and Weatherley 2000) coagulation and electrochemical techniques (Zhou and He 2007; Beltran-Heredia et al. 2011) as well as membrane processes (Baouab et al. 2000) are generally ineffective for total colour removal. Wide range of other methods has been developed, like adsorption on organic and inorganic matrices, photocatalysis, chemical oxidation, microbiological or enzymatic decomposition (Hao et al. 2000).
The removal of synthetic dyes from wastewaters is especially difficult when reactive dyes are present, for which conventional wastewater treatment plants give low removal efficiency (Kumari and Abraham 2007).
Dyeing with reactive dyes, mostly applied to cellulosic fibres like cotton, consists in the formation of a covalent bond between the dye and the fibre, under alkaline pH conditions and high temperature. The hydrolysis of the dye also occurs as a secondary reaction leading to a low degree of fixation on the fibre and considerable losses of the hydrolyzed dye in the effluent.
Retention onto solid materials has been viewed as a procedure of choice (Crini 2006) for dye removal since it is an effective and economic physical method that could allow a complete decolourization of the wastewaters and their possible re-use.
Activated carbon is a widely used and effective adsorbent, but its use is limited by the high costs associated with its regeneration or replacement. Several studies have been reported on the preparation of activated carbons from solid wastes and their application to dye removal (Kadirvelu et al. 2003; Onal 2006). Low-cost materials, in their natural and modified forms, have been also extensively studied as alternative adsorbents for dyes (Ahmad et al. 2007; Gurses et al. 2006). Among the different adsorbents, layered double hydroxides (LDHs), also known as hydrotalcite-like compounds or anionic clays, are promising waste scavengers, particularly for dye molecules (Allmann 1968). Recent studies showed that the use of LDHs for the retention of organic anions has given very good results (De Roy et al. 1992; Lakraimi et al. 2000).
The layered structure of LDHs, coupled with the high charge density of their sheets, their smooth and flexible structure, with relatively few interlayer bondings, are accountable for their important anion exchange and intercalation aptitudes allowing for the incorporation into their interlayer domains of a large number of inorganic and organic anions. The properties and applications of these compounds have been the subject of a number of general and specialized reviews (Rives 2001; Braterman et al. 2003; Zümreoglu-Karan and Ay 2012).
The aim of the present work is to assess the retention capacity of [Zn–Al–Cl] LDHs for the RB19 dye and to understand the mechanism involved. The influences of contact time, initial pH of the solutions, LDHs dose and the RB19/[Zn–Al–Cl] mass ratio have been investigated. The localization of the dye in the interlayer space and/or on external surfaces of the LDHs is studied by XRDIR and TG-DTG analyses.
Materials and methods
Adsorbent preparation and characterization
All experiments were carried out under a stream of N2 to avoid, or at least minimize contamination by atmospheric CO2. The [Zn–Al–Cl] LDH, with a [Zn]/[Al] ratio of 2, was synthesized by coprecipitation at a constant pH of 9.0 and at room temperature. Mixtures of molar ZnCl2 and AlCl3 aqueous solutions were slowly introduced into the reactor where the pH was maintained constant by the simultaneous addition of a 1.0-M NaOH solution. The resulting slurry was then stirred for 72 h at room temperature. The precipitate was filtered, washed several times with decarbonated water and then dried at room temperature (25 °C).
Experimental [Zn]/[Al] ratio in the solid and its cell parameters
d = c/3 (nm)
Physical and chemical characteristics of RB19 dye
Remazol Brilliant Blue special
Reactive blue 19
2-(3-(4-Amino-9,10-dihydro-3-sulpho-9,10-dioxoanthracen-4-yl) aminobenzenesulphonyl) vinyl)disodiumsulphate
Toxicity to fishes, CL50
Retention experiments were carried out by the batch equilibrium technique at room temperature (25 °C), at constant pH, maintained by addition of NaOH, and under a stream of N2. Amounts of [Zn–Al–Cl] were dispersed in 100-mL RB19 solutions. The initial concentration of RB19 was varied between 15 and 100 mg/L. After filtration, the solid products obtained were dried at room temperature before being analyzed by XRD and IR techniques. The supernatants were recovered and the residual dye concentration was determined by UV–Vis spectroscopy. The absorbance was measured at 594 nm on a Spectronic Genesys 20 spectrophotometer.
The XRD equipment used was a Siemens D 501 diffractometer. Samples of unoriented powder were exposed to copper Kα radiation (λ = 0.15415 nm). Measurement conditions were 2θ range 2–70°, step size: 0.08° 2θ, and step counting time: 4 s. Data acquisition was effective on a DACO-MP microcomputer. Unit cell constants were calculated using a least squares refinement.
Absorbance IR spectra were recorded on a Perkin-Elmer 16 PC spectrophotometer, at a resolution of 2 cm−1 and averaging over 100 scans, in the range 400–4,000 cm−1. Samples were pressed into KBr discs.
TG and DTG studies were performed in air with a SETARAM TG-DSC 92 instrument. Curves were recorded on 20 mg of the sample over a temperature range of up to 1,000 °C at a heating rate of 5 °C/min. The weights were corrected for the small effect of the gas flow as function of temperature.
Results and discussion
Preliminary adsorption experiments were conducted to determine the optimal conditions for the retention of RB19 on LDHs regarding the pH value, contact time (tc), initial concentration (Ci) of adsorbate and the mass ratio adsorbate /adsorbent.
Effect of pH
This phenomenon takes place despite the precautions taken during the preparation of the solid sample and the kinetics study, when the pH value is high. Following these experiments, it was decided to carry out the retention experiments at pH 7.
Effect of contact time (kinetic study)
As may be seen, the RB19 retention is inversely proportional to the mass of LDH. The Qm value increases when the LDH mass decreases; this is somehow normal if we know that retention of RB19 by the LDH can be done, in addition to adsorption, also by intercalation between the layers. This intercalation requires an increasing molar ratio (RB19/Cl) and therefore relatively small mass of LDH to allow maximum exchange of the chloride ions by the dye’s ions.
Langmuir parameters for RB19 retention by the [Zn–Al–Cl] LDH
These isotherms had an appearance reminiscent of those of the Langmuir adsorption isotherms. They provide a satisfactory linearization of retained amount, Q. This linearization was used to standardize the method of determining the maximum quantity Qm. It also provides the affinity constant, K, on which it is difficult to pronounce. In that order of size, the K values are at least comparable, suggesting that whatever the LDH mass used, the type of interaction between adsorbent and adsorbate is the same. However, the maximum adsorbed amount decreases when the mass of LDH is increased.
Furthermore, the adsorption on the surface does not affect this distance for low concentrations of the dye. By contrast, for relatively high concentrations, increased interlayer distance means that there is an exchange between chloride ions and the anions of RB19, which are larger and accompanied by the maximum occupancy of the available anionic sites.
The cell parameters for the best compound, in terms of crystallinity, were a = 0.306 nm, c = 2.775 nm and the interlayer distance d = 0.925 nm. The study of the effect of RB19 concentration indicates the existence of two phases, in terms of crystallinity, for 15 g/L (Fig. 5). The presence of the line at d = 0.778 nm indicates that the exchange is only partial in this case. In addition, the interlayer distance varies slightly with the ion concentration.
Effect of mass ratio RB19/LDH
RB19/[Zn–Al–Cl] ≤3: retention on the surface (adsorption).
RB19/[Zn–Al–Cl] >3: both adsorption and intercalation between the layers.
The retention of RB19 by the LDH attained 98 % for an RB19/LDH mass ratio of 15. This value compares well with the removal of RB19 by coconut coir activated carbon that attained 96 % (Chaudhuri et al. 2009).
TG and DTG analysis
The second weight loss at 220 °C is assigned to the interlayer water. The third weight loss around 320 °C is attributed to the dehydroxylation of the brucite-like layers. This destruction of the hydroxylated sheet is held at the same temperature as the starting matrix [Zn–Al–Cl] (Fig. 8a). These three first stages are similar to those previously reported for hydrotalcite-like materials (Cavani et al. 1991; Labajos and Rives 1996). The fourth weight loss corresponds to the departure of chloride ions for the [Zn–Al–Cl] phase in the form of gaseous HCl, which results in a signal on the DTG curve centred at 630 °C. This behaviour is identical to that of the host matrix. This confirms that the exchange of chloride ions by the RB19 is partial.
The fifth weight loss centred at about 950 °C starting from 750 °C is due to the decomposition of RB19 anion by combustion reactions, probably in the form of H2O, CO2, NO2 and SO2. The combustion of the organic anion at higher temperatures may be probably due to slow kinetics of decomposition of the sulphonate groups and may confirm the hypothesis of a slow diffusion of SO2 in the oxide matrix already formed at such temperatures (Lakraimi et al. 2006).
Anyway, this TG study confirms the retention of RB19 by [Zn–Al–Cl] and confirms the results obtained by other characterization techniques.
The present study shows that the [Zn–Al–Cl] can be used as an effective adsorbent for the removal of the RB19 dye from aqueous solutions. The LDH was able to remove up 98 % of RB19 from solutions whose initial concentration was varied between 15 and 100 mg/L. The quantity eliminated was found to depend on LDH dose, RB19/LDH mass ratio, contact time and the pH of the dye solution. The adsorption is described by Langmuir-type isotherm due to the surface homogeneity. The results of three characterization techniques, XRD, IR and TG-DTG, agree and indicate that the retention phenomenon may have taken place following two ways: adsorption and anion exchanges when the RB19/LDH mass ratio is higher than 3 and adsorption only when the ratio is low. The intercalation of the organic ion in the layered host structure was clearly evidenced by the net increase in the basal spacing from 0.778 nm for [Zn–Al–Cl] to 0.925 nm in the new phase, [Zn–Al–RB19].
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